[flang] Merge flang-compiler/f18

This is the initial merge of flang-compiler, which is done in this way
principally to preserve the history and git-blame, without generating a large
number of commits on the first-parent history of LLVM.

If you don't care about the flang history during a bisect remember that you can
supply paths to git-bisect, e.g. `git bisect start clang llvm`.

The history of f18 was rewritten to:

* Put the code under /flang/.
* Linearize the history.
* Rewrite commit messages so that issue and PR numbers point to the old repository.

Credit to Peter Waller for writing the flatten and merge script.

Updates: flang-compiler/f18#876 (submission into llvm-project)
Mailing-list: http://lists.llvm.org/pipermail/llvm-dev/2020-January/137989.html ([llvm-dev] Flang landing in the monorepo - next Monday!)
Mailing-list: http://lists.llvm.org/pipermail/llvm-dev/2019-December/137661.html ([llvm-dev] Flang landing in the monorepo)

Co-authored-by: Peter Waller <peter.waller@arm.com>

Author: DavidTruby

flang/.clang-format

@@ -0,0 +1,21 @@
+---
+# See: https://clang.llvm.org/docs/ClangFormatStyleOptions.html
+BasedOnStyle: LLVM
+AlignAfterOpenBracket: DontAlign
+AlignEscapedNewlines: DontAlign
+AlignConsecutiveAssignments: false
+AlignConsecutiveDeclarations: false
+AlignOperands: false
+AlignTrailingComments: false
+IncludeCategories:
+  - Regex:           '^<'
+    Priority:        4
+  - Regex:           '^"(llvm|llvm-c|clang|clang-c)/'
+    Priority:        3
+  - Regex:           '^"(flang|\.\.)/'
+    Priority:        2
+  - Regex:           '.*'
+    Priority:        1
+...
+
+# vim:set filetype=yaml:

flang/.drone.star

@@ -0,0 +1,60 @@
+def clang(arch):
+    return {
+        "kind": "pipeline",
+        "name": "%s-clang" % arch,
+        "steps": [
+            {
+                "name": "test",
+                "image": "ubuntu",
+                "commands": [
+                    "apt-get update && apt-get install -y clang-8 cmake ninja-build lld-8 llvm-8-dev libc++-8-dev libc++abi-8-dev libz-dev git",
+                    "git clone --depth=1 -b f18 https://github.com/flang-compiler/f18-llvm-project.git llvm-project",
+                    "mkdir llvm-project/build && cd llvm-project/build",
+                    'env CC=clang-8 CXX=clang++-8 CXXFLAGS="-stdlib=libc++" LDFLAGS="-fuse-ld=lld" cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../install -DLLVM_TARGETS_TO_BUILD=host -DLLVM_INSTALL_UTILS=On -DLLVM_ENABLE_PROJECTS="mlir" ../llvm',
+                    "ninja install",
+                    "cd ../..",
+                    "mkdir build && cd build",
+                    'env CC=clang-8 CXX=clang++-8 CXXFLAGS="-UNDEBUG -stdlib=libc++" LDFLAGS="-fuse-ld=lld" cmake -GNinja -DCMAKE_BUILD_TYPE=Release .. -DLLVM_DIR=/drone/src/llvm-project/install/lib/cmake/llvm -DMLIR_DIR=/drone/src/llvm-project/install/lib/cmake/mlir -DLLVM_EXTERNAL_LIT=/drone/src/llvm-project/build/bin/llvm-lit',
+                    "ninja -j8",
+                    "ctest --output-on-failure -j24",
+                    "ninja check-all",
+                ],
+            },
+        ],
+
+    }
+
+def gcc(arch):
+    return {
+            "kind": "pipeline",
+            "name": "%s-gcc" % arch,
+            "steps": [
+                {
+                    "name": "test",
+                    "image": "gcc",
+                    "commands": [
+			"apt-get update && apt-get install -y cmake ninja-build llvm-dev libz-dev git",
+                        "git clone --depth=1 -b f18 https://github.com/flang-compiler/f18-llvm-project.git llvm-project",
+		        "mkdir llvm-project/build && cd llvm-project/build",
+		        'env CC=gcc CXX=g++ LDFLAGS="-fuse-ld=gold" cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../install -DLLVM_TARGETS_TO_BUILD=host -DLLVM_INSTALL_UTILS=On -DLLVM_ENABLE_PROJECTS="mlir" ../llvm',
+                        "ninja install",
+                        "cd ../..",
+                        "mkdir build && cd build",
+                        'env CC=gcc CXX=g++ CXXFLAGS="-UNDEBUG" LDFLAGS="-fuse-ld=gold" cmake -GNinja -DCMAKE_BUILD_TYPE=Release .. -DLLVM_DIR=/drone/src/llvm-project/install/lib/cmake/llvm -DMLIR_DIR=/drone/src/llvm-project/install/lib/cmake/mlir -DLLVM_EXTERNAL_LIT=/drone/src/llvm-project/build/bin/llvm-lit',
+                        "ninja -j8",
+                        "ctest --output-on-failure -j24",
+                        "ninja check-all",
+                    ],
+                },
+            ],
+
+        }
+
+def main(ctx):
+    return [
+        clang("amd64"),
+        clang("arm64"),
+        gcc("amd64"),
+        gcc("arm64"),
+    ]
+

flang/.gitignore

@@ -0,0 +1,21 @@
+Debug
+Release
+MinSizeRel
+build
+root
+tags
+TAGS
+*.o
+.nfs*
+*.sw?
+*~
+*#
+CMakeCache.txt
+*/CMakeFiles/*
+*/*/CMakeFiles/*
+*/Makefile
+*/*/Makefile
+cmake_install.cmake
+formatted
+.DS_Store 
+.vs_code